This invention relates to a method for mounting mirrors such as those which might be used in a solar power plant.
Solar power plants typically comprise a large number of mirrors arranged to reflect solar radiation into a boiler placed at the center of the mirror field on top of a tower. This boiler feeds steam to a turbine located at the foot of the tower to drive an electric generator. Since the position of each mirror must be changed periodically to direct reflected sunlight onto the boiler, the mirrors are mounted on orientators which automatically adjust their position at regular intervals. By way of example a 15 MW solar power plant can comprise 1250 mirrors, each having an area of 50 square meters, and a boiler located on top of a tower 150 meters in height. The mirrors are arranged in 20 concentric semicircles having radii from 100 to 500 meters and centered on a point directly beneath the boiler. To form a focused light spot that is smaller than the boiler the shape of the mirrors must approximate that of a spherical mirror with a large radius. In the above example this radius would range from 360 to 1000 meters, depending on the distance from the mirror to the boiler.
The mirrors used in solar power plants are designed with the following considerations:
they should be relatively inexpensive, so that the solar power plant will be competitive with conventional electric power plants;
they should be reliable so as to limit replacement, maintenance and adjustment costs;
they should retain a shape close to that of the ideal convergent mirror in spite of deformations due to the wind, to heat and to their own weight;
they should have a relatively small weight to simplify the design of the orientators.
To comply with these conditions, mirrors have been designed either with a single curved glass face or with a plurality of plane glass faces which are silvered on one side to provide a reflecting surface. Typically, the back side of the glass is silvered so that the glass seals and protects the reflecting surface from the weather.
To form a single curved glass face, the glass must be cast onto an appropriate form or a hot sheet of glass must be curved by applying it to a form. Since these approaches require polishing of the glass after curving, they are generally too expensive.
In the case of mirrors having a plurality of plane glass faces, square or horizontal mirror components are set onto supports such that each facet formed by one of these components will be tangent at its center to the ideal mirror. A good approximation to the ideal mirror can thus be obtained provided a very large number of plane components are used. However, the shaping of the components is expensive; and each component must be protected against the weather over its entire periphery to prevent loosening of the silver layer from the glass. In addition, the positioning of each component is costly and must be adjusted periodically; and since it is necessary to provide a large number of bearing points on the mirror supports, the structure is relatively heavy and requires the use of fairly complex orientators.
Alternatively, it is possible to deform plane glass face components to give them an approximately spherical shape by positioning their periphery on bearings which coincide with the ideal mirror and pulling on their center. While it is theoretically possible to make mirrors having deformed glass face components that are larger than mirrors with plane faces, even stronger bearings must be used to limit the flexure of the glass face under wind pressure and its own weight. In addition to the other drawbacks of multi-component mirrors, it is also necessary to paste an anchor onto the rear silvered face of each component or to make a hole in it so as to exert a pulling action during deformation. This, however, increases the risk of breakage during use, and makes it necessary to provide thermal treatment to the glass face to limit such risks.
Typical mirror constructions of the prior art are disclosed in such patents as U.S. Pat. Nos. 1,951,404 and 3,884,217 and French Pat. Nos. 447,329, 1,035,832 and 1,238,883.